I need help wiht a problem

[philip augustus]

I need some help with some pyhsics problems, ANY help on just one of these problems would be greatly apprceiated, I'll give rep points to anybody who can help me (note I'n not askign for the answers, jsut some help on how I shuld figure this out)

A car traveling at 124 km/hr hits it brakes and decelerates at the rate of 4.8 m/s2. How far does the car travel while stopping?

I used the equation VF^2= 2a (delta) X + Vi^2
but I keep on getting the wrong answer, and I know its the only equation that I can use for this kind of problem.



A car traveling down the road increases its speed from 45 km/hr to 81 km/hr over a distance of 517 m. What is the acceleration of the car?

A race car starts from rest and accelerates at the rate of 6.5 m/s2 for 1050 m. The car then decelerates at the rate of 3.4 m/s2 until it stops. What is the total distance that the car travels?

A brick is dropped out of a window. It hits the ground 1.55 seconds later. How high is the window?

You throw a ball straight up into the air. It returns to the ground 5.11 seconds later. What was the initial speed of the ball?

While exploring the canopy of the rainforest in equatorial South America, Anastasia falls from a branch 28.0 meters high. Exactly 1.1 seconds later Joe falls from the same branch. How high above the ground is Joe when Anastasia splats into the mud below? (the mud is very squishy and nobody is injured)

Marie shoots an arrow vertically upward at a sandhill crane (she misses the crane, so you PETA members can relax). If the arrow travels to a maximum height of 86.0 meters, calculate how fast it was shot.

And, for how much time was Marie's arrow in the air?

[ajm317]

This should go in the homework thread.

What numbers are you plugging into the equation and where? I can't tell you what you're doing wrong unless you tell me what you're doing.

[philip augustus]

sorry I didnt know we had a homework thred, the numbers I'm putting in are. I first find out the time it takes for it to reach final velocity, which I use VF=VI+ A (delta)T which when I put the numbers in I use 0=124+-4.8xT, when I do that I get T=25.83, then I use the other equation and the numbers I put in are X=124x25.83+.5(-4.8x124^2). I put the wong equation up first lol.

[ajm317]

I put the wong equation up first lol.

Actually you can solve the problem in 1 step instead of 2 using the equation you posted the first time.

when I put the numbers in I use 0=124+-4.8xT

That's your first problem there.

124 is in km/hr. 4.8 is in m/s^2. Plugging them into the equation as they are doesn't make any sense. It's like saying "ajm317 traveled 20 feet, Simetrical traveled 20 m, so together they traveled 40 feet." It doesn't work that way. You need to either convert m/s^2 into km/hr^2, in which case your time will be in hours, or convert km/hr into m/s in which case your time will be in seconds.

X=124x25.83+.5(-4.8x124^2)

The equation you are using here is x=vo*t+.5*a*t^2. You don't want to square the velocity, you want to square the time. Always try to keep track of units, when you are adding 2 things of different units you've made a mistake. For example here you were adding km and m*km^2/(s^2*hr^2). If you do step one by converting km/hr into m/s and put time in the t^2 term you'll be adding m's with m's.

[philip augustus]

so my main problem was that I wasnt converting my units? cuz if thats true I didnt really do that for my other problems either. which would explain a lot.

[ajm317]

so my main problem was that I wasnt converting my units? cuz if thats true I didnt really do that for my other problems either. which would explain a lot.

Yeah, it's a very common mistake people make when they're taking their early Physics courses. They're used to math, where you don't have to worry about units.

[Mathias]

A simple rule to remember: To convert km/h into m/s, divide by 3.6 to convert m/s into km/h, multiply by 3.6

I see that some of the problems involve gravity, but makes no mention of much gravity accelerates an object, so in case you don't know; it's 9.81 (-9.81 of course, if the object is moving upwards). Pointless of me to mention it really, as your physics books also says so.

[philip augustus]

thanks I'll remember that

[philip augustus]

I've figured most of them out by now, only three of them are giving me a hard time

[Mathias]

I've figured most of them out by now, only three of them are giving me a hard time

Which ones? I can help you, we have been working with this in physics in school.

[drak10687]

hmm, really? Well, not so in my "junior college" (its actually Cegep, since I live in Quebec) but maybe it will be different once I get to University. I actually prefer this method from my "math habits" as its a lot more helpful when you're doing calculus to leave things in "exact" numbers... but in our college labs, our accuracy is just too low most of the time.

[Simetrical]

This seems a very general statement, which isn't true of all areas of maths; I can think of several examples of where units are important in maths off the top of my head.

Only for applications of math. Units are irrelevant to the math itself, in almost all cases. Of course, you could observe that different units are sometimes used for angles, for instance, but angles are viewed as dimensionless (pure numbers) in both math and physics. Otherwise something like sin 45° would make no sense in Taylor series expansion: 45° + (45°)³/3! + ... is already inconsistent, if degrees are a unit. Other than angles, there's nothing in math itself that I'd say even approaches a unit.

Of course, pretty much all applied math courses teach applications, which may involve units. In my experience, units are ignored there: you're given consistent units to start with and just discard them when you do the problem. That may be different in Norway, but I should note that units are still only relevant to applications of math, not to math itself. (Incommensurable quantities are relevant to math, but those do not by themselves constitute units.)

It is true that g₀ (or g_n), the standard acceleration due to gravity (the acceleration at sea level at 45.5^o latitude), is roughly 9.81ms^-2 (as mentioned earlier the units are vital), but it is important to remember that this is just an approximation. A more accurate value is 9.80665ms^-2.

A more accurate value than 9.8 m/s² (9.81 is already over-accurate) does not exist for the acceleration due to gravity at sea level, because that acceleration varies from about 9.78 to 9.83 m/s² depending on where you are on the Earth's surface, giving it an error of ±0.025 m/s² or so and thus only two significant figures. My introductory physics textbook, noting this fact, used 9.8 m/s² as g throughout, expanding its accuracy to 9.80 m/s² when it wanted three significant figures.

The figure you cite, "standard gravity", is a basically arbitrary standard that should have no bearing on physics problems that are meant to reflect the real world, unless the problem happens to occur at a very precise latitude and altitude that happens to exactly correspond to the one chosen for the standard. Standard gravity may be used, arbitrarily, for things like calibrating scales that might be used anywhere in the world, but it's probably not ideal for that either.